Material handling vehicles such as lift trucks are used to pick up and deliver loads between stations. A typical lift truck 10 has a mast 12, which supports a load-lifting carriage 14 that can be raised along the mast 12 (see FIG. 1). The carriage 14 typically has one or more carriage bars 16 to which a fork frame 18 is mounted. The carriage bars 16 are coupled to the mast in a way that allows the lift truck 10 to move the carriage bars 16 up and down, but not laterally relative to the truck. The fork frame 18 carries a pair of forks 20. A driver of the lift truck 10 maneuvers the forks 20 beneath a load prior to lifting it. In the course of moving various loads, the drivers may need to change the lateral position of the forks 20 relative to each other. To change the lateral position of the forks 20 on the lift truck 10 shown in FIG. 1, the driver must dismount from the lift truck 10 and manually reposition the forks 20.
A powered fork positioner allows the driver of a lift truck so equipped to change the lateral position of the forks without dismounting from the lift truck. This can save effort, time and money. A typical fork positioner uses hydraulic actuators mounted to the fork frame to move the forks relative to the fork frame.
A fork positioner may be combined with a side shifter. A typical side shifter uses hydraulics for laterally displacing the fork frame with respect to the center line of the lift truck. A hydraulic actuator connecting the truck carriage to the fork frame provides the shifting action. A side shifter is useful in situations where the driver has the correct distance between the forks set, but needs to have a precise alignment between the forks and the pockets in the load that are available to receive the forks. Pallets carrying loads have very wide pockets in the pallet for receiving forks. For working with pallets, precise alignment of the forks with the load is not necessary. Other loads may only have pockets that are not much wider than the forks, requiring precise alignment. A driver may not be able to reliably align the forks with the pockets using the whole lift truck, necessitating backing up the entire lift truck and trying again until alignment is achieved. A side shifter allows the driver to make small adjustments in aligning the forks with the load if an approach with the entire lift truck fails to align the forks with the load adequately.
Fork positioners and side shifters are well known, but existing designs connect the forks to hydraulic actuators with an exposed actuator rod. The exposed actuator rod is vulnerable to damage from contact with loads and other objects in their normal work environment. The actuator rod needs to have a smooth finish to pass through the seals in the hydraulic actuator. Even small nicks in the surface of the actuator rod can damage the seals when the actuator rod is withdrawn into the actuator. This can necessitate more frequent replacement of seals, leading to higher maintenance costs and more down time for the lift truck.
Additionally, the exposed actuator rod not only transmits the force for moving a fork from the actuator to the fork, but also any moment generated by the application of the force to the fork. Moment is generated in the actuator rod because the force applied to a fork usually causes a deflection in the fork (unless the fork is secured so that it cannot rotate even the slightest degree and is perfectly rigid or secured at fulcrums equidistant from the center of where the force is applied—conditions that can almost never be arranged in a practical forklift). The force is not applied to the fork at a single point, but over a finite area. Even if the force is evenly distributed over that area, when a portion of that area is deflected more than other portions of that area, the distribution of the force over the area becomes uneven, causing a moment to be transmitted back into the object applying the force—i.e., the actuator rod. This moment causes a deflection in the actuator rod, which if severe enough, can cause a permanent distortion in the actuator rod. An actuator rod with a distortion can damage the seals of the actuator when the rod is drawn in and may even jam in the actuator. The amount of deflection caused by a given amount of moment is increased as the length of the actuator rod is increased. Designers of fork positioners have had to take these considerations into account and usually have responded by making the actuator rod thicker than would be necessary to transmit the required force just to make the rod more resistant to deflection caused by moment. This necessitates a larger polished surface for passing through the seals of the actuator and larger seals as well.